This invention relates to manufacturing of sodium hydroxide. More particularly, the invention relates to manufacturing of potassium hydroxide and potassium sulfate by electrolyzing sodium sulfate.
Demonstrated worldwide demand for some sodium-based chemicals, particularly for sodium hydroxide (caustic soda), has been on the rise in recent years. This strong demand, which is forecast to continue, keeps this chemical in tight supply position, thereby holding the price at a high level. This trend is not the same with respect to all sodium-based chemicals. In particular, the demand for sodium sulfate and, as a consequence, the price of this chemical is declining at the same time as the demand for caustic soda is rising.
This declining trend in the demand for and prices of sodium sulfate combined with the strong demand for and relatively high prices of other sodium-based chemicals, in particular of caustic soda, created a need for a simple and economical process for producing sodium hydroxide from sodium sulfate as feedstock. This need is even more strongly perceived in countries endowed with vast natural resources of sodium sulfate. This is, for example, the case in Canada, which has large deposits of natural sodium sulfate located in Southern Saskatchewan.
The most direct process for producing sodium hydroxide from sodium sulfate is the electrolytic conversion of an aqueous solution of sodium sulfate into aqueous solutions of sulfuric acid and caustic soda. Numerous implementations of this process are known in the prior art. Most of them make use of electrolytic cells employing diaphragms or ion permeable membranes to separate the product solutions from the feed solution, thus avoiding contamination of the products by the feedstock material.
U.S. Pat. No. 2,829,095, issued Apr. 1, 1958, to Oda et al., discloses a process for the production of acidic and alkaline solutions by electrolysis of a salt solution in a multi-compartment electrolytic cell partitioned by a plurality of anion and cation exchange membranes. The patent also discloses the use of the process for direct production of sodium hydroxide and sulfuric acid from Glauber""s salt (sodium sulfate decahydrate).
U.S. Pat. Nos. 3,135,673, issued Jun. 2, 1964, to Tirrell et al., and 3,222,267, issued Dec. 7, 1965, to Tirrell et al. claim a method and apparatus for converting aqueous electrolytic salt solutions to their corresponding acid and base solutions. A three or four compartment electrolytic cell separated by a cation exchange membrane and one or two porous, non-selective diaphragms is used for this purpose. When a solution of sodium sulfate is used as the salt solution, solutions of sodium hydroxide and sulfuric acid or sodium bisulfate are produced.
U.S. Pat. No. 3,398,069, issued Aug. 20, 1968, to Juda, claims a process for the electrolysis of an aqueous saline electrolyte in a multicellular device having cells separated by gas permeable electrodes and further partitioned by microporous fluid-permeable diaphragms or ion-permselective membranes. When applied to a solution of sodium sulfate, the process produces solutions of sodium hydroxide and sulfuric acid.
U.S. Pat. No. 3,907,654, issued Sep. 23, 1975, to Radd et al., discloses an electrolytic cell particularly useful in electrolysis of sodium sulfate to form sulfuric acid and sodium hydroxide. The cell, which does not employ any ion permeable membranes, comprises a housing having a parent solution chamber and two electrode compartments located on the lower side of the housing and separated from each other but in communication with the parent solution chamber and positioned vertically beneath or above. Mounted within the electrode compartments are an anode and a cathode, each of which is porous to permit passage of a product solution therethrough. The product solutions of sodium hydroxide and sulfuric acid separated by gravity forces are withdrawn through the porous electrodes.
U.S. Pat. No. 4,561,945, issued Dec. 31, 1985, to Coker et al., claims a process for producing sulfuric acid and caustic soda by electrolysis of an alkali metal sulfate in a three-compartment membrane cell having a hydrogen depolarized anode. Hydrogen gas in the anode chamber is oxidized to produce hydrogen cations which migrate to the central (buffer) chamber through a membrane and combine with the sulfate anions from the alkali metal sulfate solution to produce sulfuric acid. Alkali metal ions are transported across another membrane to the cathode chamber to produce caustic and gaseous hydrogen. Both membranes used in the cell are cation selective membranes.
A similar process for increasing concentration of sulfuric acid in solutions containing an alkali metal sulfate, sulfuric acid and alkaline earth metal ions is disclosed in U.S. Pat. No. 4,613,416, issued Sep. 23, 1986, to Kau et al. Also in this case the anode compartment and the cathode compartment of a three-compartment cell are each bounded by cation exchange membranes.
In U.S. Pat. No. 5,445,717 issued to Karki et al., issued Aug. 29, 1995, there is disclosed a method for the simultaneous production of alkali metal or ammonium peroxodisulphate salts and alkali metal hydroxide. In the reference, the electrolytic phase of the method is performed in a three-compartment electrolytic cell with the middle space conducting alkali metal sulfate, the anode space ammonium or alkali metal sulfate or a mixture thereof and into the cathode space water diluted alkali metal hydroxide.
This patent proceeds according to a different process to that described herein and does not provide for the preparation of potassium sulfate from a sodium sulfate starting material.
A further variation on electrosynthesis is demonstrated in Toomey, U.S. Pat. No. 5,290,404, issued Mar. 1, 1994. In this reference, an electrochemical cell is employed for producing an alcohol or carboxylic acid from a corresponding metal salt. Metal cations and residues are also recovered during the process. This process does not employ a three-compartment desalination cell, but rather employs a standard two compartment cell divided by cation permeable membrane.
In U.S. Pat. No. 5,246,551, issued Sep. 21, 1993, to Pletcher et al., an electrochemical method for the production of alkali metal hydroxides without co-production of chlorine is disclosed. In this reference, the use of a specific group of salts such as alkali metal carbonates, alkali metal bicarbonates and the like are electrolyzed in a single membrane-two solution cell with hydrogen consuming anodes. One of the advantages of this reference is the lack of the co-production of chlorine, however, the use of these specific salts is not necessary and has been overcome by the instant application.
Martin, in U.S. Pat. No. 5.230,779, issued Jul. 27, 1993, provides an electrochemical process for the production of sodium hydroxide and sulfuric acid from acidified sodium sulfate solutions. The process particularly takes place in a two compartment electrolytic cell and does not provide any teachings with respect to potassium sulfate generation, potassium chloride or ammonium sulfate generation.
Other references generally related to electrolysis and electrosynthesis include U.S. Pat. Nos. 4,033,842 and 5,286,354.
It would be desirable if there were a process where electrosynthesis or other electrochemical methods could be employed to produce useful potassium compounds such as potassium sulfate, potassium chloride as well as fertilizer compositions, namely ammonium sulfate. The present invention employs additional unit operations onto existing processes to result in the preparation of these desirable compounds.
One object of the present invention is to provide an improved electrochemical process for preparation of potassium sulfate and potassium hydroxide.
A further object of one embodiment of the present invention is to provide a process for producing potassium sulfate in an electrolytic cell, comprising the steps of:
passing a solution of sodium sulfate through a central compartment of a three-compartment electrolytic cell having a cathode compartment and an anode compartment separated from the central compartment by a cation selective ion exchange membrane and an anion selective ion exchange membrane, respectively;
passing a catholyte through the cathode compartment and an anolyte through the anode compartment;
passing a direct electric current between an anode and a cathode located in the anode compartment and the cathode compartment, respectively, thus producing sodium hydroxide in the cathode compartment and sulfuric acid in the anode compartment;
generating ammonium sulfate;
elevating the pH of the ammonium sulfate product;
introducing a potassium chloride; and
precipitating potassium sulfate.
The unit operation patented in U.S. Pat. No. 5,098,532, is a particularly useful unit operation and it has been found that by augmenting this unit operation with further process steps, useful products such as potassium sulfate, potassium chloride, ammonium sulfate and potassium hydroxide all can be easily formed without concomitant production of chlorine.
As was proposed in the prior art, metal hydroxides have been previously synthesized electrochemically, but the prior art references failed to provide a process where sodium sulfate could be used as an initial feedstock to prepare the compounds indicated above.
It is known that sodium sulfate can be split into caustic and ammonium sulfate in a three-compartment electrochemical cell by the teachings of U.S. Pat. No. 5,098,532. This technology, as well as other technologies often encounter technical challenges created by the concentrations of acids and basis formed in the electrochemical cell and the back migration of protons across the anion exchange membrane. These problems manifest in negative effect on the current efficiency. The acid concentration in the anolyte also contributes to accelerated anode degradation.
In the instant invention, these problems are alleviated by the introduction of ammonia into the anolyte compartment. This produces caustic with ammonium sulfate the ladder commonly used as fertilizer. The fertilizer (ammonium sulfate) can then be upgraded by chemical transformation in the electrochemical cell to potassium sulfate.
In accordance with a further object of one embodiment of the present invention there is provided a process for producing potassium hydroxide and potassium sulfate, the process comprising the steps of:
(a) providing a first and a second three-compartment electrolytic cell, each cell having a cathode compartment and an anode compartment separated from a central compartment by a cation selective ion exchange membrane and anion selective ion exchange membrane;
(b) passing a solution of sodium sulfate through the central compartment;
(c) passing a catholyte through the cathode compartment and an anolyte through the anode compartment of the first three-compartment electrolytic cell;
(d) passing a direct current between an anode and a cathode located in the anode compartment and the cathode compartment, respectively, to produce sodium hydroxide in the cathode compartment and sulfuric acid in the anode compartment;
(e) generating ammonium sulfate;
(f) elevating the pH of the ammonium sulfate;
(g) introducing potassium chloride into the ammonium sulfate;
(h) forming potassium sulfate;
(i) feeding a solution of the potassium sulfate into the central compartment of a second three-compartment electrolytic cell;
(j) elevating the pH of said potassium sulfate; and
(k) forming potassium hydroxide.
To carry out the process according to the invention, any electrolytic flow cell using a two or three-compartment configuration can be used in either continuous or batch mode of operation.
In the process, the anolyte, the catholyte and the feed solution are circulated through the respective compartments of the cell at a flow rate depending on the cell used, typically of from about 0.1 L/min to about 30 L/min. The current density is limited by the efficiency considerations (current efficiency of the process decreases with growing current density) and by the stability of the membranes used. Typical current densities are in a range of from about 1 mA/cm2 to about 500 mA/cm2.
The feed solution of sodium sulfate may have a concentration of from about 0.1M to the solubility limit. The concentration of from about 1M to about 1.1M is preferred. For concentrated feed solutions, it may be necessary to heat the solution generally to about 60xc2x0 C. prior to circulating it through the cell, to prevent the crystallization of the salt.
The feed solution should be as free as possible of heavy metal contaminants that are usually present in the naturally occurring Glauber""s salt. If this salt is used as a starting material, the bulk of heavy metal ions can be precipitated, for example, by addition of sodium carbonate and/or sodium hydroxide to a solution of the salt. The remaining amounts of polyvalent cations, in particular of calcium and magnesium ions, can be removed by treating the resulting solution with an ion exchange resin, e.g. by passing the solution through an ion exchange column packed with a suitable ion exchange material, for example Duolite(trademark) C-467 from Rohm and Haas, or an equivalent material. After such a treatment the heavy metal ion concentration normally will not exceed about 20 ppb.
The catholyte and the anolyte can both be water, but it is preferred that they are solutions of sodium hydroxide and ammonium sulfate, respectively, as this gives improved conductivity. In the case of sodium hydroxide solution, the starting concentration should be in the range of from about 0.01M to about 2.74M. In the case of ammonium sulfate solution the starting concentrations should be in the range of from about 0.01M to about 3.5M. A concentration of about 3M is preferred. The choice of the starting concentrations of the anolyte and the catholyte may be also affected by the mode of operation of the electrolytic cell. For example, for the continuous mode of operation, starting concentrations closer to the upper limits of the above ranges are preferred.
To avoid an excessive accumulation of hydrogen ions in the anolyte, ammonia in either the liquid or the gaseous form is introduced into the anolyte at such a rate as to keep the pH of the solution at a predetermined level. The choice of suitable pH of the anolyte may be affected by several other factors, in particular by the ion exchange membranes and anode materials used. Generally, the pH of the anolyte may be maintained at any level in the range of from about 0.5 to about 12. A pH of from about 0.5 to about 7 is preferred and pH of from about 0.5 to about 3.5 is particularly preferred. It appears that under these acidic conditions there is little or no anode corrosion as well as no or very little formation of nitrogen and ammonium nitrate due to electrooxidation of ammonia.
The materials for electrodes, beside providing good current conduction, must be corrosion resistant under the operating conditions of the cell. Suitable cathodes are low hydrogen over potential cathodes, for example gold, platinum, nickel or stainless steel. Because of the lower cost, nickel and stainless steel are preferred.
The choice of the anode material is mostly restricted by the presence of ammonia in the anolyte solution. Under conditions (pH 9 to 12) cathode made of some materials, such as nickel, graphite and stainless steel may corrode quickly. In this range of pH anodes made of platinum, platinized titanium, magnetite or anodes of low oxygen over potential such as DSA(trademark) type electrodes (iridium or platinum oxides on a titanium substrate) are preferred. Under acidic conditions (pH 0.5 to 2) DSA-O2 anodes are preferred. However, less expensive materials, such as lead dioxide on titanium or Ebonex(trademark) (material comprising Ti4 O7) may be used. Lead dioxide on lead would be even less expensive anode material, but there exists a possibility that this material might liberate lead into the anolyte, thus making ammonium sulfate unacceptable for use as a fertilizer.
The ion-selective membranes used to separate the anode and cathode compartments from the central compartment are essentially insoluble, synthetic, polymeric organic ion-exchange resins in sheet form. Those selective to cations usually have sulfonate and/or carboxylate groups bound to the polymers; those selective to anions usually have amino functionality bound to the polymer. These ion exchange membranes are commercially available under various trade names, for example Nafion(trademark) or Flemion(trademark) (cation exchange membranes) or Neosepta(trademark) (anion exchange membranes). Cation selective membranes made of stable perfluorinated cation exchange resins are preferred.
Even though, in principle, any cation or anion exchange membrane may be used in the process according to the invention, their choice may be in practice limited to those showing sufficiently good stability under operating conditions of the electrolytic cell. For example, the choice of the anion selective membrane maybe limited by both the concentration of sulfate ion and/or ammonia in the anolyte and the presence of hydroxyl ions in the feed solution, due to the back migration of hydroxyl ions from the catholyte. Of the membranes showing good stability, membranes having high ionic selectivity and low electrical resistance are preferred. A person skilled in the art will be able to choose suitable membranes without difficulty.
An example of the anion exchange membrane preferred for carrying out the process of the invention is Neosepta(trademark) AMH membrane, which shows good stability at the anolyte pH in a range of 1-12. Examples of preferred cation exchange membranes are perfluorinated membranes such as Nafion(trademark) and Flemion(trademark) membranes, which show good stability for NaOH concentration up to 50%.
Having thus generally described the invention, reference will now be made to the accompanying drawings.